Select the letter-cluster that can replace the question mark (?) in the given letter-cluster series.

POUR, MLRO, IHNK, DCIF, ?

XWCZ

Explanation:

POUR, MLRO, IHNK, DCIF, ?

The logic followed here is :

P → (-3) → M → (-4) → I → (-5) → D → (-6) → X

O → (-3) → L → (-4) → H → (-5) → C → (-6) → W 

U → (-3) → R → (-4) → N → (-5) → I → (-6) → C

R → (-3) → O → (-4) → K → (-5) → F → (-6) → Z

So, the letter-cluster that replaces the (?) to complete the series is "XWCZ"